/*
 *  Licensed under the Apache License, Version 2.0 (the "License");
 *  you may not use this file except in compliance with the License.
 *  You may obtain a copy of the License at 
 * 
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 */
using System;

namespace org.apache.commons.pool
{

    /**
     * A pooling interface.
     * <p>
     * <code>ObjectPool</code> defines a trivially simple pooling interface. The only
     * required methods are {@link #borrowObject borrowObject}, {@link #returnObject returnObject}
     * and {@link #invalidateObject invalidateObject}.
     * </p>
     * <p>
     * Example of use:
     * <pre style="border:solid thin; padding: 1ex;"
     * > Object obj = <code style="color:#00C">null</code>;
     *
     * <code style="color:#00C">try</code> {
     *     obj = pool.borrowObject();
     *     <code style="color:#00C">try</code> {
     *         <code style="color:#0C0">//...use the object...</code>
     *     } <code style="color:#00C">catch</code>(Exception e) {
     *         <code style="color:#0C0">// invalidate the object</code>
     *         pool.invalidateObject(obj);
     *         <code style="color:#0C0">// do not return the object to the pool twice</code>
     *         obj = <code style="color:#00C">null</code>;
     *     } <code style="color:#00C">finally</code> {
     *         <code style="color:#0C0">// make sure the object is returned to the pool</code>
     *         <code style="color:#00C">if</code>(<code style="color:#00C">null</code> != obj) {
     *             pool.returnObject(obj);
     *        }
     *     }
     * } <code style="color:#00C">catch</code>(Exception e) {
     *       <code style="color:#0C0">// failed to borrow an object</code>
     * }</pre>
     * </p>
     *
     * <p>See {@link BaseObjectPool} for a simple base implementation.</p>
     *
     * @param <T> the type of objects held in this pool
     * 
     * @author Rodney Waldhoff
     * @author Sandy McArthur
     * @version $Revision: 1222396 $ $Date: 2011-12-22 14:02:25 -0500 (Thu, 22 Dec 2011) $
     * @see PoolableObjectFactory
     * @see ObjectPoolFactory
     * @see KeyedObjectPool
     * @see BaseObjectPool
     * @since Pool 1.0
     */
    public interface ObjectPool<T>
    {
        /**
         * Obtains an instance from this pool.
         * <p>
         * Instances returned from this method will have been either newly created with
         * {@link PoolableObjectFactory#makeObject makeObject} or will be a previously idle object and
         * have been activated with {@link PoolableObjectFactory#activateObject activateObject} and
         * then validated with {@link PoolableObjectFactory#validateObject validateObject}.
         * </p>
         * <p>
         * By contract, clients <strong>must</strong> return the borrowed instance using
         * {@link #returnObject returnObject}, {@link #invalidateObject invalidateObject}, or a related method
         * as defined in an implementation or sub-interface.
         * </p>
         * <p>
         * The behaviour of this method when the pool has been exhausted
         * is not strictly specified (although it may be specified by implementations).
         * Older versions of this method would return <code>null</code> to indicate exhaustion,
         * newer versions are encouraged to throw a {@link NoSuchElementException}.
         * </p>
         *
         * @return an instance from this pool.
         * @throws IllegalStateException after {@link #close close} has been called on this pool.
         * @throws Exception when {@link PoolableObjectFactory#makeObject makeObject} throws an exception.
         * @throws NoSuchElementException when the pool is exhausted and cannot or will not return another instance.
         */
        T borrowObject();// throws Exception, NoSuchElementException, IllegalStateException;

        /**
         * Return an instance to the pool.
         * By contract, <code>obj</code> <strong>must</strong> have been obtained
         * using {@link #borrowObject() borrowObject}
         * or a related method as defined in an implementation
         * or sub-interface.
         *
         * @param obj a {@link #borrowObject borrowed} instance to be returned.
         * @throws Exception 
         */
        void returnObject(T obj);// throws Exception;

        /**
         * <p>Invalidates an object from the pool.</p>
         * 
         * <p>By contract, <code>obj</code> <strong>must</strong> have been obtained
         * using {@link #borrowObject borrowObject} or a related method as defined in
         * an implementation or sub-interface.</p>
         *
         * <p>This method should be used when an object that has been borrowed
         * is determined (due to an exception or other problem) to be invalid.</p>
         *
         * @param obj a {@link #borrowObject borrowed} instance to be disposed.
         * @throws Exception
         */
        void invalidateObject(T obj);// throws Exception;

        /**
         * Create an object using the {@link PoolableObjectFactory factory} or other
         * implementation dependent mechanism, passivate it, and then place it in the idle object pool.
         * <code>addObject</code> is useful for "pre-loading" a pool with idle objects.
         * (Optional operation).
         *
         * @throws Exception when {@link PoolableObjectFactory#makeObject} fails.
         * @throws IllegalStateException after {@link #close} has been called on this pool.
         * @throws UnsupportedOperationException when this pool cannot add new idle objects.
         */
        void addObject();// throws Exception, IllegalStateException, UnsupportedOperationException;

        /**
         * Return the number of instances
         * currently idle in this pool (optional operation).
         * This may be considered an approximation of the number
         * of objects that can be {@link #borrowObject borrowed}
         * without creating any new instances.
         * Returns a negative value if this information is not available.
         *
         * @return the number of instances currently idle in this pool or a negative value if unsupported
         * @throws UnsupportedOperationException <strong>deprecated</strong>: if this implementation does not support the operation
         */
        int getNumIdle();// throws UnsupportedOperationException;

        /**
         * Return the number of instances
         * currently borrowed from this pool
         * (optional operation).
         * Returns a negative value if this information is not available.
         *
         * @return the number of instances currently borrowed from this pool or a negative value if unsupported
         * @throws UnsupportedOperationException <strong>deprecated</strong>: if this implementation does not support the operation
         */
        int getNumActive();// throws UnsupportedOperationException;

        /**
         * Clears any objects sitting idle in the pool, releasing any
         * associated resources (optional operation).
         * Idle objects cleared must be {@link PoolableObjectFactory#destroyObject(Object) destroyed}.
         *
         * @throws UnsupportedOperationException if this implementation does not support the operation
         */
        void clear();// throws Exception, UnsupportedOperationException;

        /**
         * Close this pool, and free any resources associated with it.
         * <p>
         * Calling {@link #addObject} or {@link #borrowObject} after invoking
         * this method on a pool will cause them to throw an
         * {@link IllegalStateException}.
         * </p>
         *
         * @throws Exception <strong>deprecated</strong>: implementations should silently fail if not all resources can be freed.
         */
        void close();// throws Exception;

        /**
         * Sets the {@link PoolableObjectFactory factory} this pool uses
         * to create new instances (optional operation). Trying to change
         * the <code>factory</code> after a pool has been used will frequently
         * throw an {@link UnsupportedOperationException}. It is up to the pool
         * implementation to determine when it is acceptable to call this method.
         *
         * @param factory the {@link PoolableObjectFactory} used to create new instances.
         * @throws IllegalStateException when the factory cannot be set at this time
         * @throws UnsupportedOperationException if this implementation does not support the operation
         * [Obsolete] to be removed in pool 2.0
         */
        [Obsolete]
        void setFactory(PoolableObjectFactory<T> factory);// throws IllegalStateException, UnsupportedOperationException;
    }
}